Field of the Invention
The present invention relates to a heat-treating method for improving the wear- and corrosion-resistance of a chromium-plated steel substrate. More particularly, the present invention relates to a method for improving the wear- and corrosion-resistance of a chromium-plated steel substrate by heat-treating the chromium-plated steel substrate under optimum conditions to inhibit a drop in corrosion-resistance of a steel substrate due to fine cracks formed in a chromium layer, and to improve a hardness of the chromium layer as well.
In general, chromium platings, which have been widely used in various industrial and decorative fields, are largely classified into two categories: a hexa-valent chromium plating process and a tri-valent chromium plating process. The hexa-valent chromium plating process has been mainly applied since 1920s, while the tri-valent chromium plating process was recently developed to reduce environmental pollution.
In particular, since the chromium platings used for various industrial fields have excellent resistance to corrosion and wear, they are currently used in moldings, automobiles and various machine parts. In addition, decorative chromium platings exhibit higher excellent resistance to corrosion than any other metal plating.
The hexa-valent chromium plating is advantageous in terms of low-cost treatment and excellent resistance to corrosion, wear and heat. However, fine cracks are likely to occur in a hexa-valent chromium layer in view of properties intrinsic to the chromium plating. The fine cracks lower a resistance of the steel substrate to corrosion, which results in the steel substrate being rusted.
On the other hand, a tri-valent chromium layer exhibits a hardness similar to the hexa-valent chromium layer. Like the hexa-valent chromium layer, the tri-valent chromium layer has a low resistance to corrosion due to the presence of fine cracks formed therein. The hardness of the hexa-valent chromium layer is reduced by a relaxed residual stress inside the layer at high temperature. However, the hardness of the tri-valent chromium layer is increased by a small amount of carbon present in an organic complexing agent of a plating bath. That is, the carbon is incorporated into the tri-valent chromium layer, and the Cr—C platings thus formed are crystallized at high temperature to form highly hard chromium carbide precipitates such as Cr7C3, Cr23C6, etc. The precipitates increase the corrosion-resistance of the trivalent chromium layer. At this time, increased hardness due to precipitation of stable carbides improves wear-resistance of the tri-valent chromium layer.
Although the hardness and corrosion-resistance of the tri-valent chromium layer are increased as described above, corrosion-resistance of a tri-valent chromium-plated steel substrate is decreased due to the presence of fine cracks formed at the tri-valent chromium layer.
Therefore, the present invention has been made in view of the above problems, and it is an object of the present invention to provide a heat-treating method for improving the wear- or corrosion-resistance of a chromium-plated steel substrate by plating hexa-valent or tri-valent chromium onto a steel substrate, followed by oxidizing the surface of an steel substrate exposed to the air through fine cracks formed at a chromium layer to form Fe3O4 layers on the surface of the steel substrate. In accordance with the heat-treating method, the corrosion-resistance of hexa-valent chromium-plated steel substrate is improved, and the corrosion- and wear-resistance of tri-valent chromium-plated steel substrate are improved.
To achieve the above objective, there is provided a heat-treating method for improving the wear- and corrosion-resistance of a chromium-plated steel substrate, comprising the steps of:                plating a chromium layer onto an steel substrate; and        heating the chromium-plated steel substrate in an oxidizing gas environment at above atmospheric pressure to form oxidized layers containing magnetite (Fe3O4) on the surface of the steel substrate, the surface of the steel substrate being partly exposed to the air through penetrating cracks formed in the chromium layer.        
In accordance with the heat-treating method according to the present invention, the step of heating in an oxidizing gas environment to form oxidized layers is preferably carried out at 180˜570° C. for 10˜600 minutes.
The oxidized layers formed by heating in an oxidizing gas environment preferably contain magnetite (Fe3O4) in an amount of above 60% by weight.
The oxidizing gas used in the present invention is preferably steam, carbon dioxide, air, or mixtures thereof.
In accordance with one aspect of the present invention, the heat-treating method for improving the wear- and corrosion-resistance of the chromium-plated steel substrate further comprises the step of carbonitriding the chromium-plated steel substrate prior to heating in an oxidizing gas environment to form oxidized layers. The carbonitriding step permits formation of iron nitrides (Fe2-3N) in the form of a ε-phase on the surface of the steel substrate exposed to the air through fine cracks formed in the chromium layer.